Hi macpark
The truth is you wouldn't have saved up the money you have spent on postage stamps.
I am in the same position as you, in that I send off at least a dozen crosswords each week, but I have had limited success over the years, which has covered my costs.
My view is that if I have spent the time in completing a crossword, it is worth the cost of a stamp with the possibility that I may be rewarded for doing so.
I don't know if you have won, but I'm sure it will happen. As somebody once said to me, "You definitely won't win if you don't send it off".
Maybe your Premium Bond win is a sign of things to come!
Incidentally, did you know that you can access the NS&I site from about the 3rd working day of the month, enter your Premium Bond reference number, and you will be told whether or not you have won that month.
The best of luck, and keep plugging away.
Regards,
WTW